The dependence of phytoplankton photosynthesis on light intensity may be altered by the range and frequency of variations in light intensity recentlv experienced by the organisms. A major source of the fluctuations in light intensity experienced by phytoplankton in the upper ocean is vertical motion. We estimate time and space scales for \Tertical displacements of phytoplankton caused by turbulent mixing, internal waves, Langmuir circulations, and double diffusive processes. In the surface layer, depending on windspeed, current shear and stratification, we find that time scales for cycling of phytoplankton by turbulent eddies and mixing vary from about 0.5 h to hundreds of hours for vertical displacements of the order of 10 m. In the seasonal thermocline, turbulent diffusive time scales for displacements as small as several meters are weeks to months, whereas similar displacements by internal waves occur over periods of several minutes to several hours, according to the strength of the density stratification, and are then dominant. Langmuir cells seem to scale as the large turbulent eddies and need not be treated separately, and double diffusive processes seem to be of minor importance. The formulation used here of a vertical turbulent diffusion coefficient K, as a function of observable quantities-e the rate of dissipation of turbulent kinetic energy, and N the local buoyancy frequency-should also be us&d for estimating vertical fluxes of nutrients. In addition, this formulation is reversible in time and can be used to estimate the recent depth and light history of phytoplankton taken from the upper ocean.

The validity of the assumption of local isotropy is investigated using measurements of three orthogonal components of the turbulent velocity fields associated with initially high-Reynolds-number geophysical turbulence. The turbulent fields, generated by various large-scale internal motions caused by tidal flows over an estuarine sill, decay under the influence of stable mean density gradients. With measurements from sensors mounted on a submersible, we examine the evolution of spectral shapes and of ratios of cross-stream to streamwise components, as well as the degree of high-wavenumber universality, for the observational range of the parameter I≡ ks/kb = lb/ls. This ratio is a measure of separation between the Kolmogoroff wavenumber ks≡ (ε/ν3)¼ ≡ 2π/ls typical of scales by which turbulent kinetic energy has been dissipated (at rate ε), and the buoyancy wavenumber kb ≡ (N3/ε)½ ≡ 2π/lb typical of scales at which the ambient stratification parameter N ≡ (−gρz/ρ0)½ becomes important. For values of I larger than ∼ 3000, inertial subranges are observed in all spectra, and the spectral ratio ϕ22/ϕ11 of cross-stream to streamwise spectral densities reaches the isotropic value of 4/3 for about a decade in wavenumber. As ks/kb decreases, inertial subranges vanish, but spectra of the cross-stream and streamwise components continue to satisfy isotropic relationships at dissipation wavenumbers. We provide a criterion for when ε may safely be estimated from a single measured component of the dissipation tensor, and also explore questions of appropriate low-wavenumber normalization for buoyancy-modified turbulence.

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The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data.The finestructure of density profiles is described using the Thorpe scales L T . It is shown from microstructure data taken in the bottom water that the Ozmidov scale L O is related to L T by the linear relationship L O ϭ 0.95L T , similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 ϫ 10 Ϫ4 m 2 s Ϫ1 is found for the bottom water.Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4ЊC isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.8ЊC isotherm, emphasizing the role of these passages for bottom-water property modifications.Corresponding author address: Bruno Ferron, IFREMER, Laboratoire de Physique des Océans, B.P. 70,

The diffusive regime of double-diffusive convection is reviewed, with a particular focus on issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not easily represented in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.